This invention relates to devices for extracting from a double-binary phase-shift keyed (PSK) signal the clock frequency of the base band signal thereof.
Four-phase PSK transmission systems have previously been used for transmission of digital information. Recently a double-binary PSK system has been proposed as a system which is resistant to the deteriorating effects of a nonlinear transmission channel. Modulation of this type are a modification of the conventional four-phase PSK system in which the modulated signal is formed using two base band signals which are out of phase by an amount of time corresponding to half the length of the time slot. For details of this modulation system, reference may be had, for example, to the article, "The Effects of Filtering and Limiting a Double-Binary PSK Signal,"by Robert K. Kwan, "IEEE Transactions on Aerospace and Electronic Systems," Vol. AES-5, No. 4, July 1969, pp. 589-594. In a digital phase-modulation system, the clock signal must be extracted from the modulated wave at the receiving end of the system to provide time slot information. However, many of the clock extraction devices previously used in a four-phase digital PSK system (in which principally no phase change is effected every half time slot) cannot be used, without modication, in a double-binary PSK system.
For a double-binary PSK system, a method of clock extraction may be contemplated in which the modulated signal is frequency-doubled and, from the resulting signal having a frequency of 2f.sub.c, two signals, 2f.sub.c +f.sub.s and 2f.sub.c -f.sub.s, both including the clock frequency f.sub.s, are separated out. The two signals so separated are mixed to produce a signal 2f.sub.s and, by halving the frequency of this signal, the clock frequency f.sub.s is readily obtainable. Such clock extraction, however, inherently involves a phase ambiguity of 180.degree. since the clock is regenerated through a 1/2 frequency divider, and again the clock regenerated cannot be utilized immediately as it is.